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Computational model of abdominal aortic aneurysm inception and evolution
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.ORCID iD: 0000-0002-2749-3381
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Incidence of abdominal aortic aneurysm (AAA) is increasing in the aging society of the western world. Development of AAA is mostly asymptomatic and is characterized by a bulge in the abdominal aorta. However, AAA may suddenly rupture, which results in an internal bleeding associated with a high mortality rate. Patients with AAA undergo regular screening until treatment indication. To date, statistical criteria are used to decide whether the risk of rupture exceeds the risk of intervention. Models of AAA development help to understand the disease progression and to yield patient-specific criterion for AAA rupture.

Up to date, sophisticated models of AAA development exist. These models assume the abdominal aorta as a thin-walled structure, which saves the computational effort. This thesis aims at investigating the importance of employing a thick-walled model of the aorta. The effects on AAA development that cannot be captured with a thin-walled model are of interest. In Paper A, the thick-walled model of growth and remodeling of one layer of a AAA slice has been extended to a two-layered model. The parameter study has been performed to investigate the influence of mechanical properties and growth and remodeling (G&R) parameters of two individual layers on the gross mechanical response and G&R of the artery. It was concluded that the adventitia acts to protect the arterial wall against rupture even in pathological state.

In Paper B, the model was extended to an organ level model of AAA development. Furthermore, the model was incorporated into a so-called Fluid-Solid-Growth (FSG) framework, where the AAA development is loosely coupled to the blood flow conditions such as wall shear stress. One patient-specific geometry of the abdominal aorta is used to illustrate the model capabilities. A transmurally non-uniform distribution of the strains of individual arterial constituents was observed. In addition, an increased aneurysm tortuosity was observed in comparison to a thin-walled approach. These findings signify the importance of a thick-walled approach to model the aneurysm development. Finally, the proposed methodology provides a realistic basis to further explore the growth and remodeling of AAA on a patient-specific basis.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2014. , viii, 8 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0553
Keyword [en]
aneurysm, three dimensional, elastin degradation, growth, remodeling, fluid-solid-growth model
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-142649ISBN: 978-91-7595-065-5 (print)OAI: oai:DiVA.org:kth-142649DiVA: diva2:703982
Presentation
2014-03-19, Seminarierummet, Teknikringen 8d, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20140311

Available from: 2014-03-11 Created: 2014-03-10 Last updated: 2014-03-11Bibliographically approved
List of papers
1. Influence of differing material properties in media and adventitia on arterial adaptation: application to aneurysm formation and rupture
Open this publication in new window or tab >>Influence of differing material properties in media and adventitia on arterial adaptation: application to aneurysm formation and rupture
Show others...
2013 (English)In: Computer Methods in Biomechanics and Biomedical Engineering, ISSN 1025-5842, E-ISSN 1476-8259, Vol. 16, no 1, 33-53 p.Article in journal (Refereed) Published
Abstract [en]

Experimental and computational studies suggest a substantial variation in the mechanical responses and collagen fibre orientations of the two structurally important layers of the arterial wall. Some observe the adventitia to be an order of magnitude stiffer than the media whilst others claim the opposite. Furthermore, studies show that molecular metabolisms may differ substantially in each layer. Following a literature review that juxtaposes the differing layer-specific results we create a range of different hypothetical arteries: (1) with different elastic responses, (2) different fibre orientations, and (3) different metabolic activities during adaptation. We use a finite element model to investigate the effects of those on: (1) the stress response in homeostasis; (2) the time course of arterial adaptation; and (3) an acute increase in luminal pressure due to a stressful event and its influence on the likelihood of aneurysm rupture. Interestingly, for all hypothetical cases considered, we observe that the adventitia acts to protect the wall against rupture by keeping stresses in the media and adventitia below experimentally observed ultimate strength values. Significantly, this conclusion holds true in pathological conditions.

Keyword
media, adventitia, layer-specific, growth, arterial adaptation, abdominal aneurysm
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-142646 (URN)10.1080/10255842.2011.603309 (DOI)000316056100004 ()2-s2.0-84872580188 (Scopus ID)
Note

QC 20140311

Available from: 2014-03-10 Created: 2014-03-10 Last updated: 2017-12-05Bibliographically approved
2. A thick-walled fluid–solid–growth model of abdominal aortic aneurysm evolution: Application to a patient-specific geometry
Open this publication in new window or tab >>A thick-walled fluid–solid–growth model of abdominal aortic aneurysm evolution: Application to a patient-specific geometry
2014 (English)Report (Other academic)
Abstract [en]

We propose a model for abdominal aortic aneurysms that considers the wall (solid), the blood (fluid) and the wall growth within a three-dimensional finite element framework. The arterial wall is considered as a thick-walled nonlinearly elastic circular cylindrical tube consisting of two layers corresponding to the media-intima and adventitia, where each layer is treated as a fiber-reinforced material with the fibers corresponding to the collagenous component. The blood is modeled as a Newtonian fluid with constant density and viscosity; no slip and no-flux conditions are applied at the arterial wall. The metabolic activity in the arterial wall is reflected by elastin degradation which is coupled with the level of wall shear stress, while the collagen fiber network is continuously remodeled in the artery such that the collagen fiber strain tends towards a homeostatic strain. The computational framework consists of a structural FE-solver (CMISS), a fluid solver using a finite volume formulation and additional routines which pass the aneurysm geometry to the fluid solver and feeds CMISS with the information on the blood flow conditions. One illustrative patient-specific geometry of an abdominal aortic wall is discretized with hexahedral finite elements and the fluid domain is generated by an unstructured tetrahedral mesh with prism layers lining the boundary. The evolution of wall shear stress and elastin degradation is investigated over a time period of 10 years; the influence of transmurally non-uniform elastin degradation is analyzed. The results show that both the elastin and the collagen strains can become transmurally non-uniform during the aneurysm development. This effect cannot be captured by membrane formulations. The proposed methodology provides a realistic basis to further explore the development of patient-specific aneurysmal disease.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. 34 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 552
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-142648 (URN)
Funder
Swedish Research Council
Note

QC 20140311

Available from: 2014-03-10 Created: 2014-03-10 Last updated: 2014-03-11Bibliographically approved

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